TOUGH times for embattled Bruce Ratner? The Atlantic Yards developer has already replaced architect Frank Gehry and whittled down the cost of his $1 billion Nets arena by $200 million. Now he’s sold his Montauk estate for $10 million to Chelsea art dealer David Zwirner, reports The Post’s Jennifer Gould Keil. The 4,500-square-foot house on 7 acres on Old Montauk Highway was designed by classic East End architect Francis Fleetwood. Prudential Douglas Elliman broker John Golden had no comment. Ratner, we hear, has been looking in Quogue –for something less pricey, on the ocean and closer to the city.